Rotor for a hover-capable aircraft and related method of control

ABSTRACT

A rotor for a hover-capable aircraft includes a hub rotatable about a first axis and at least two blades hinged to the hub. Each blade has a main portion hinged to the hub and a tip portion, which is arranged radially outermost with respect to first axis with respect to the corresponding main portion. The tip portion of each blade is movable with respect to the corresponding main portion of that blade. The tip portion of each blade is selectively movable with respect to the corresponding main portion of that blade between a first position, in which it defines a dihedral or anhedral angle with respect to the corresponding main portion; and a second position, in which it defines a positive or negative sweep angle with respect to the corresponding main portion.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a U.S. National Phase Application under 35 U.S.C. §371 of International Patent Application No. PCT/EP2017/072,496, filedSep. 7, 2017, which claims the priority of European Application No.16187546.3, filed Sep. 7, 2016, which is incorporated by reference as ifexpressly set forth in its entirety herein.

TECHNICAL FIELD

The present invention relates to a rotor for a hover-capable aircraft.

The present invention also relates to a method of controlling a rotorfor a hover-capable aircraft.

BACKGROUND ART

Known hover-capable aircrafts, such as helicopters for example, comprisea fuselage, a main rotor positioned on the top of a central portion ofthe fuselage and an anti-torque tail rotor with the function ofcountering the torque generated by the main rotor on the fuselage.

The main rotor basically comprises a mast rotatable about an axis, a hubcoupled to this mast, and a plurality of blades fastened in a cantilevermanner to the above-mentioned hub.

In particular, each blade has a substantially longitudinal extensionradial to the axis of the mast and, in use, is driven in rotation by thehub on a motion plane transversal to the axis of the mast.

In addition, each blade is movable with respect to the hub on some orall of the orientation planes so as to enable the different manoeuvresof the helicopter.

Each blade comprises, in particular:

-   -   a main portion defining a root of the blade, which is hinged to        the hub; and    -   a tip portion, which delimits the blade at the far end from the        mast with respect to the main portion.

The main portion of each blade is longer than the corresponding tipportion.

In the helicopter industry, it is known to shape the tip portions of theblades with an anhedral (negative dihedral) angle with respect to thecorresponding main portions. In other words, the tip portions of theblades are inclined downwards from the corresponding main portions andtowards the fuselage of the helicopter.

Thanks to the presence of an anhedral angle, it is possible to improvethe figure of merit in the helicopter's hovering conditions.

Despite enabling the behaviour of the helicopter to be improved inhovering conditions, the use of tip portions having anhedral anglesmakes the helicopter noisier in forward flight conditions.

In the industry, there is a need to make rotors for hover-capableaircraft that enable both preserving aerodynamic efficiency in hoveringconditions and reducing noise in forward flight conditions.

More specifically, there is a need to make rotors of theabove-identified type while containing as far as possible thedisplacement of the barycentre of the tip portions of the blades. Thisis in order to avoid having to counter high loads due to centrifugalacceleration, which can reach several hundreds of g on the tip portions.

In addition, there is a need to make rotors of the above-identified typethat employ moderate actuating forces and/or without unbalancing therotor in the event of erroneous control of one of the blades and/orwithout altering the aerodynamic profile of the blades, and thereforewithout penalizing the overall aerodynamic efficiency of the helicopter.

US2016/0075430 describes a rotor for a hover-capable aircraft in whicheach blade of the rotor comprises a tip portion hinged to a main rootportion, and which can be operated to adjust the anhedral angleaccording to the flight conditions of the aircraft.

The rotor shown in US2016/0075430 further comprises a hydraulic actuatorfor adjusting the anhedral angle of the tip portion.

However, the use of power operated actuators for controlling theanhedral angle of the tip portion leaves room for improvement.

In particular, it is difficult to house an actuator within thesmall-sized region between the main blade portion and the tip portion.

Furthermore, the tip portions are subjected to very high aerodynamic andinertial loads, in particular centrifugal forces. Accordingly, theactuating system requires very powerful actuating forces and torques.

A need is therefore also felt within the industry to selectively adjustthe angle between the tip portion and main portion, which requires aslittle as possible the use of power operated actuators.

DISCLOSURE OF INVENTION

The object of the present invention is to produce a rotor for anaircraft capable of hovering that satisfies at least one of theabove-specified needs in a simple and inexpensive manner.

The aforesaid object is achieved by the present invention, in so far asit relates to a rotor for a hover-capable aircraft, as claimed in claim1.

The present invention also relates to a method of controlling a rotorfor a hover-capable aircraft, as claimed in claim 19.

The present invention also relates to a rotor for a hover-capableaircraft, as claimed in claim 24, 35 or 37.

The present invention also relates to a method of controlling a rotorfor a hover-capable aircraft, as claimed in claim 32, 36 or 47.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, four preferredembodiments are described hereinafter, purely by way of non-limitativeexample and with reference to the accompanying drawings, in which:

FIG. 1 is a front view of a helicopter comprising a rotor according to afirst embodiment of the present invention equipped with blades arrangedin a first operating condition of hovering and in a second operatingcondition of forward flight;

FIG. 2 is a top view of the helicopter in FIG. 1 showing the blades inthe first and second operating conditions;

FIG. 3 is a perspective view on a highly enlarged scale of a blade ofthe rotor of the helicopter in FIGS. 1 and 2, with parts removed forclarity;

FIG. 4 is a view taken from one side of the helicopter and on a stillfurther enlarged scale of the blade in FIG. 3 in the first operatingcondition, with parts removed for clarity;

FIG. 5 is a view taken from above the helicopter of the blade in FIG. 4in the first operating condition;

FIG. 6 is a view taken from the front of the helicopter of the blade inFIGS. 4 and 5 in the first operating condition;

FIG. 7 is a view taken from one side of the helicopter and on a stillfurther enlarged scale of the blade in FIG. 3 in the second operatingcondition;

FIG. 8 is a view taken from above the helicopter of the blade in FIG. 7in the first operating condition;

FIG. 9 is a view taken from the front of the helicopter of the blade inFIGS. 7 and 8 in the first operating condition;

FIG. 10 is a perspective view of the blade in FIGS. 4 to 9 in the firstoperating condition, with parts removed for clarity;

FIG. 11 is a perspective view of the blade in FIGS. 4 to 10 in thesecond operating condition, with parts removed for clarity;

FIG. 12 is a cross-section of the blade in FIGS. 4 to 11 along lineXII-XII in FIG. 11;

FIG. 13 is a longitudinal section of the blade in FIGS. 4 to 12 alongline XIII-XIII in FIG. 11;

FIGS. 14 and 15 are cross-sections of the blade in FIGS. 4 to 14 alonglines XIV-XIV and XV-XV in FIG. 13, respectively;

FIG. 16 shows a helicopter comprising a rotor according to a secondembodiment of the present invention;

FIG. 17 shows a perspective view, on a highly enlarged scale, of a bladeof the rotor in FIG. 16, with parts removed for clarity; and

FIGS. 18 to 20 schematically show the shape of the blade of a rotoraccording to a third embodiment of the invention, with parts removed forclarity;

FIG. 21 shows a blade of a rotor according to a fourth embodiment of thepresent invention in a first configuration;

FIGS. 22 and 23 a are partially sectioned views of the blade of FIG. 21in a first position and in a second position respectively, in enlargedview and parts removed for clarity;

FIG. 23b is an enlarged view of some components of FIG. 23 a;

FIGS. 24 to 26 are sections taken along respective lines XXIV-XXIV andXXV-XXV and XXVI-XXVI of FIG. 22;

FIGS. 27 to 29 show the temporal variation of the lift acting on the tipportions of the blade of FIGS. 21 to 23 a, when the helicopter isrespectively in hover, in forward flight at a speed below a thresholdvalue and at a speed above the threshold value;

FIG. 30 shows the aerodynamic moment acting on the tip of the blade ofFIGS. 21 to 23 a during the transition from the first position to thesecond position in the condition of FIG. 27;

FIG. 31 shows the aerodynamic moment acting on the tip of the blade ofFIGS. 21 to 23 a during the transition from the second position to thefirst position in the condition of FIG. 27;

FIG. 32 shows the movement of the tip between the first and the secondposition as a function of the forward speed of the helicopter;

FIGS. 33 and 34 show the lift distribution along the span of the bladeof FIGS. 21 to 23 a in a solid line with reference to a forward flightand in dotted line with reference to an hovering condition, respectivelyfor a retreating blade and an advancing blade;

FIG. 35 shows the forces acting on the tip portion of the blade of FIGS.21 to 23 a together with further components of this blade;

FIG. 36 is a partially sectioned view of a blade of a rotor according toa fifth embodiment of the present invention in a first position;

FIG. 37 is a perspective view of a tip portion of the blade of FIG. 36,with parts removed for clarity and in an enlarged view; and

FIG. 38 is a section taken along lines XXXVIII-XXXVIII of FIG. 36.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIGS. 1 and 2, reference numeral 1 indicates a helicopter.The helicopter 1 basically comprises a fuselage 2, a main rotor 3rotating about an axis and a tail rotor 4 located at one end of thefuselage 2 and rotating about its own axis transversal to axis A. Inparticular, the anti-torque tail rotor 4 projects in a cantilever mannerfrom a fin situated at a tail end of the fuselage 2 and is designed tocounter the torque transmitted by the rotor 3 to the fuselage 2.

In greater detail, the rotor 3 basically comprises:

-   -   a mast 5 rotating about an axis A transversal to the axis of        rotation of the main rotor;    -   a plurality of blades 6, five in the case shown, extending along        respective directions substantially radial to axis A; and    -   a hub 7 operatively connected to the mast 5 and from which the        blades 6 extend in a cantilever manner.

In greater detail, the hub 7 drives the blades 6 in rotation about axisA and enables the blades 6 to rotate, under the action of an externaldrive, about respective directions of extension to vary the respectiveangles of attack with respect to the airflow.

The blades 6 are hinged to the hub 7 so as to be movable about differentaxes, according to the configuration of the rotor 3.

In the following description, reference will be made to only one blade6, as the blades 6 are all identical.

The blade 6 comprises:

-   -   a main portion 10 defining a root of the blade 6 hinged to the        hub 7 and extending along an axis C (FIGS. 3 to 8) transversal        to axis A; and    -   a tip portion 11 arranged at the far end of the main portion 10        with respect to the tip portion 11.

The main portion 10 is longer than the tip portion 11 in the directionof radial extension of the blade 6.

The tip portion 11 is movable with respect to the main portion 10.

Advantageously, the tip portion 11 is selectively movable with respectto the main portion 10 between:

-   -   a first position (assumed by the blades indicated with a        continuous line in FIG. 1 and by reference numeral 6 b in FIG.        2), in which it defines a dihedral or anhedral angle α with        respect to the main portion 10; and    -   a second position (assumed by the blades indicated by with a        dotted line in FIG. 1 and by reference numeral 6 a in FIG. 2),        in which it defines a positive or negative sweep angle β with        respect to the main portion 10.

The main portion 10 and tip portion 11 have respective leading 12 and 13and trailing edges 14 and 15.

The term dihedral/anhedral angle α indicates the angle formed betweenthe leading edge 13 of the tip portion 11 and the leading edge 12 of themain portion 10 on a plane parallel to axis A and to axis C, at thecommon point of the leading edges 12 and 13.

An anhedral angle α indicates that the leading edge 13 of the tipportion 11 is inclined towards the fuselage 2 with respect to theleading edge 12 of the main portion 10.

Whereas a dihedral angle α indicates that the leading edge 13 of the tipportion 11 is inclined away from fuselage 2 with respect to the leadingedge 12 of the main portion 10.

The term sweep angle β indicates the angle formed between the leadingedge 13 of the tip portion 11 and the leading edge 12 of the mainportion 10 in a plane transversal to axis A, at the common point of theleading edges 12 and 13.

A positive sweep angle β indicates that the leading edge 13 of the tipportion 11 is arranged downstream with respect to the leading edge 12 ofthe main portion 10, with reference to the direction of motion of theblade 6 about axis A.

Whereas a negative sweep angle β indicates that the leading edge 13 ofthe tip portion 11 is arranged upstream with respect to the leading edge12 of the main portion 10, with reference to the direction of motion ofthe blade 6 about axis A.

More specifically, the sweep angle β of the tip portion 11 with respectto the main portion 10 is minimized when the tip portion 11 is arrangedin the first position.

The dihedral angle α of the tip portion 11 with respect to the mainportion 10 is zero, when the tip portion 11 is arranged in the secondposition.

The blade 6 also comprises an actuator unit 16 to produce the transitionof the tip portion 11 between the first and second positions.

In greater detail, the actuator unit 16 comprises (FIGS. 3 to 9):

-   -   a motor 20;    -   a plurality of rods 21 operatively connected to consecutive        sections of the tip portion 11 and destined to induce the        transition of the tip portion 11 between the first and second        positions; and    -   a transmission unit 22 functionally interposed between the motor        20 and the rods 21.

In the case shown, the motor 20 is a stepper motor.

Furthermore, the motor 20 is housed inside the main portion 10.

The rods 21 rotate about respective axes parallel to each other and toaxis C, and in turn comprise:

-   -   sections 25 housed in the main portion 10 and extending parallel        to the associated axis C; and    -   sections 26 housed in the tip portion 11 and extending along        respective axes D with respect to axis C.

In particular, sections 26 are inclined with respect to sections 25.

In the case shown, sections 25 and the associated axes C are straight,while sections 26 and the associated axes D are curved.

In the case shown, sections 26 are shaped in such a way that:

-   -   the axes D extend, with respect to axis C, towards the fuselage        2 when the blade 6 is arranged in the first position (FIGS. 4 to        6);    -   the axes extend, with respect to axis C, in the opposite        direction to that of the motion of the blade 6 when the latter        is arranged in the second position (FIGS. 7 to 9).

Sections 26 are housed in a rotatable manner inside respective apertures27 defined by ribs 28 of the tip portion 11.

In this way, the rods 21 are rotatable with respect to the ribs 28.

Preferably, the rods 21 are connected to the respective ribs 28 bycorresponding articulated joints 29 (FIGS. 4-9).

Furthermore, the rods 21 are axially constrained to the ribs 28 parallelto axis C in a manner not shown in detail in the accompanying figures.

Thanks to the mode of constraint between the rods 21 and the ribs 28,the latter move in a plane orthogonal to axis C and their section inthis plane does not change during the transition of the tip portion 11between the corresponding first and second positions, so as not to alterthe sections transversal to axis D of the tip portion 11. Whereas thesections of the tip portion 11 interposed between the ribs 28elastically shear deform in a plane orthogonal to the tip portion 11.

The ribs 28 are spaced apart along the direction of extension of the tipportion 11.

More specifically, the section area of the rods 21 transversal toassociated axis D decreases when proceeding along the tip portion 11away from axis A.

The rods 21 also perform the function of supporting the bending momentsand shear loads on the associated blades 6. In other words, the rods 21perform the structural function normally carried out by the spars usedin the blades.

In the case shown in FIG. 13, the rods 21 have a circular section in aplane orthogonal to axes C and D. The diameter of the rods 21 decreaseswhen proceeding along the tip portion 11 following axes D.

In the case shown, there are five rods 21 (only some of which are shownin FIGS. 3 to 9).

The transmission unit 22 comprises, in particular, (FIGS. 3 to 9):

-   -   a cogwheel 30 connected to an output shaft 31 of the motor 20;    -   a plurality of cogwheels 32 connected to respective rods 21; and    -   a plurality of cogwheels 33 interposed between cogwheel 30 and        cogwheels 32.

The helicopter 1 also comprises a control unit 40 (only shownschematically in FIG. 3) configured to:

-   -   arrange the blades 6 in the respective first positions, when the        helicopter 1 is in the hovering condition; and    -   arrange the blades 6 in the respective second positions, when        the helicopter 1 is in the forward flight condition.

The tip portion 11 comprises a covering 50 defining the aerodynamicsurface.

The operation of the rotor 3 will now be described in detail below.

In particular, the operation of the rotor 3 is described with referenceto a single blade 6, the operation of all the blades 6 being identical.

In the case where it is necessary to maintain the helicopter 1 inhovering conditions, the control unit 40 controls the motor 20 so as toarrange the tip portion 11 in the first position (FIGS. 4 to 6), whereangle α is anhedral and the sweep angle β is minimized.

In the case where it is required to fly forwards with the helicopter 1,the control unit 40 controls the motor 20 so as to arrange the tipportion 11 in the second position (FIGS. 7 to 9), where the dihedralangle α is zero and the sweep angle β is positive.

The operation of the rotor 3 during the transition of the blade 6 fromthe first position to the second position will now be described.

The motor 20, via the transmission unit 22, causes the rotation of therods 21 about axis C, by an angle of ninety degrees in the case shown.

This causes a ninety-degree rotation with a circumferential arctrajectory of sections 26 of the rods 21 about axis C.

Since sections 26 of the rods 21 are inclined with respect to theassociated sections 25 and can rotate inside the apertures 27 of theribs 28, the tip portion 11 deforms.

In particular, the ribs 28 of the tip portion 11 move rigidly,maintaining their shape in planes orthogonal to the axes D as they arein contact with the rods 21, while the sections of the tip portion 11interposed between the ribs 28 elastically shear deform.

Referring to FIGS. 16 and 17, reference numeral 3′ indicates, as awhole, a rotor according to a second embodiment of the presentinvention.

Rotor 3′ differs from rotor 3 in that it comprises a pair of freewheels51′ interposed along one of the actuating rods 21′ and configured toprevent rotation of the rods 21′ about axis C in the clockwise andanticlockwise directions, respectively. In particular, it is possible toselectively activate one of the freewheels 51′ and deactivate the otherfreewheel 51′.

The operation of rotor 3′ differs from rotor 3 in that it uses, at leastin forward flight at speeds above a threshold value, the aerodynamicforce acting on the tip portions 11 to move them between the first andsecond operating positions.

In fact, at speeds above the threshold value, the lift acting on the tipportions 11 varies in sign depending on whether the associated blades 6are advancing or retroceding.

Referring to FIG. 16, lift is directed downwards when the blades 6 areadvancing—i.e. they have respective tangential velocities concordantwith the direction of flight of the helicopter 1—and upwards when theblades 6 are retroceding—i.e. they have respective tangential velocitiesdiscordant with respect to the direction of flight of the helicopter 1.

Rotor 3′ uses, jointly with the motor 20, the downward (upward) liftforces to arrange the tip portions 11 in the first position with ananhedral (dihedral) angle α, before setting the helicopter 1 in hoveringconditions.

In particular, when it is necessary to arrange the tip portion with adihedral (anhedral) angle α, the freewheel 51′ that allows rotation inthe clockwise (anticlockwise) direction of the rods 21′ is activated andthe other freewheel 51′ is deactivated (and vice versa).

The covering 50 also comprises (FIG. 17) fibres arranged along theassociated axes D so that it becomes axially rigid and shear-flexible.

Referring to FIGS. 18 to 20, reference numeral 3″ indicates, as a whole,a rotor according to a third embodiment of the present invention.

Rotor 3″ differs from rotor 3 in that the tip portion 11″ of each blade6 (shown entirely schematically) is formed by a plurality of elements55″ in a rigid material and elements in a viscoelastic material 56″lying on respective planes orthogonal to axis C and alternating with oneanother along axis C.

In this way, the tip portions 11 are particularly rigid to flexure inplanes orthogonal to axis C so as to maintain the shape of the ribs 28and therefore the aerodynamic efficiency of the covering 50, and sheardeformable in planes perpendicular to axis C under the action of therods 21.

The operation of rotor 3″ is identical to that of rotor 3 and istherefore not described in detail.

Referring to FIGS. 21 to 35, reference numeral 3′″ indicates, as awhole, a rotor according to a fourth embodiment of the presentinvention.

Rotor 3′″ differs from rotor 3 in that it does not comprise actuatingunit 16 and in that the adjustment of the position of tip portions 11′″relative to main portion 10 is achieved by means of the resultantmoments Mr generated by inertia forces and aerodynamic forces and/orelastic forces and/or damping forces on respective tip portions 11′″.

Furthermore, differently from blade 6 of rotor 3, 3′, 3″, the sweepangle of tip portion 11′″ remains constant as blade 6′″ moves betweenthe first and the second position.

In particular, tip portion 11′″ of each blade 6′″ is movable withrespect to relative main portion 10 between:

-   -   a relative first angular position, in which it defines anhedral        angle α with respect to relative main portion 10; and    -   a relative second angular position, in which it defines a null        angle or a minimized anhedral angle α with respect to relative        main portion 10.

In the embodiment shown, when set in the first angular position, tipportion 11′″ is arranged at a lower level than in the second angularposition.

Blade 6′″ comprises connecting means 60′″ for connecting tip portion11′″ to main portion 10 movably between the first and second position.

In greater detail, connecting means 60′″ comprises a hinge 61′″extending about an axis F tangential to axis A and about which tipportion 11′″ is hinged to main portion 10.

Still more precisely, hinge 61′″ comprises (FIG. 22):

-   -   a tubular element 105′″ coaxial to axis F;    -   a number of protrusion 106′″, 107′″, protruding from main        portion 10 and axially spaced along axis F, and angularly and        axially connected to tubular element 105′″; and    -   a plurality of joining elements 109 b′″, three in the embodiment        shown, articulated onto tubular element 105′″ about axis F and        protruding from tip portion 11′″.

Protrusion 106′″ comprises a conical end 108′″ coaxial to axis C andfitted inside a body 110′″ onto which a further joining element 109 a′″is articulated about axis F. Further joining element 109 a′″ alsoprotrudes from tip portion 11′″.

Advantageously, connecting means 60′″ can be selectively set in a firstconfiguration (FIG. 22) in which they:

-   -   allow the rotation of tip portion 11′″ with respect to main        portion 10 in a first angular direction and up to the first        angular position; and    -   prevent the rotation of tip portion 11′″ with respect to main        portion 10 in a second angular direction opposite to the first        angular direction.

Furthermore, connecting means 60′″ can be selectively set in a secondconfiguration (FIG. 23a ), in which they:

-   -   allow the rotation of tip portion 11′″ with respect to main        portion 10 in the second angular direction and up to the second        angular position; and    -   prevent the rotation of tip portion 11′″ with respect to main        portion 10 in the first angular direction.

In the embodiment shown, the first angular direction corresponds to adownwards movement of tip portions 11′″ about relative axes C, i.e. toan increase of the anhedral angle α or to a decrease of dihedral angle.

The second angular direction corresponds to an upwards movement of tipportions 11′″, i.e. to a decrease of anhedral angle α or to an increaseof dihedral angle.

Preferably, each tip portion 11′″ is set in the respective first angularposition, when helicopter 1 is in hover and is set in the respectivesecond angular position, when helicopter 1 is in forward flight.

Furthermore, each tip portion 11′″ is movable from the first angularposition to the second angular position, when helicopter 1 is in hoveror in forward flight with a speed lower than a threshold value andhelicopter 1 must be arranged in a configuration optimized for forwardflight. Accordingly, connecting means 60′″ are set in the firstconfiguration, when helicopter 1 is in hover or in forward flight with aspeed lower than a threshold value, and are moved to the secondconfiguration when helicopter 1 must be arranged in a configurationoptimized for forward flight.

Each tip portion 11′″ is also movable from the second angular positionto the first angular position when helicopter 1 is in forward flight andhelicopter 1 must be arranged in a configuration optimized for hovering.Accordingly, connecting means 60′″ are set in the second configuration,when helicopter 1 is in forward flight with a speed greater than athreshold value, and are moved to the first configuration, whenhelicopter 1 must be arranged in a configuration optimized for hovering.

In the following of the present description, reference will be made toonly one blade 6′″, being all blades 6′″ identical one another.

Preferably, blade 6′″ comprises (FIGS. 22, 23 a and 35) a rotationalspring 90′″, which is interposed between relative main portion 10 andtip portion 11′″ and exerts an elastic torque Mk about relative axis Fon tip portion 11′″.

In the embodiment shown, spring 90′″ elastically pre-loads tip portion11′″ towards the first angular position.

Preferably, blade 6′″ comprises (FIGS. 22, 23 a and 35) a rotationaldamper 92′″, which is interposed between main portion 10 and tip portion11′″, and exerts a damping torque Md dependent on the rate of rotationabout axis F on tip portion 11′″.

In the embodiment shown, spring 90′″ and/or damper 92′″ are housedinside body 110′″.

Preferably, blade 6′″ comprises a plurality of ballasts 91′″ arranged ontip portion 11′″ and aimed to locate the centre of mass of tip portion11′″ as close as possible to axis F, so as to minimize the moment Mc dueto centrifugal force and other inertial actions acting on tip portion11′″ (FIG. 35).

Ballasts 91′″ are fixed to main portion 10. In particular, ballasts 91′″are radially opposed radially to axis F with respect to respectivejoining elements 109 a′″, 109 b′″. In the embodiment shown, ballasts91′″ are made in tungsten.

With reference to FIGS. 27 to 29, it is shown the temporal variation ofthe lift acting on tip portion 11′″ for three different flight conditionof helicopter 1.

In the following of the present description, the expression “positivelift” will indicate an upwardly directed lift while the expression“negative lift” will indicate a downward directed lift.

In particular, FIG. 27 shows the temporal variation of lift resulting ontip portion 11′″ of blade 6′″ in a hover condition of helicopter 1. Inthis hover condition, the lift is positive and substantially constant invalue. Accordingly, the aerodynamic moment Mlift generated on tipportion 11′″ by the lift is constant directed in the second angulardirection, counter-clockwise in FIG. 35.

FIG. 28 shows the temporal variation of lift resulting on tip portion11′″ of blade 6′″ in a forward flight of helicopter 1 with a speed belowa threshold value. In this condition, the lift is positive butcyclically changes in value. Accordingly, the aerodynamic moment Mliftgenerated by the lift on tip portion 11′″ is variable and directed inthe second angular direction, counter-clockwise in FIG. 35.

FIG. 29 is relative to a forward flight condition of helicopter 1 with aspeed above a threshold value. In this condition, the lift acting on tipportion 11′″ is cyclically positive and negative (upwards and downwardsdirected) and changes in value. In particular (see FIGS. 33 and 34):

-   -   when blade 6′″ is advancing (FIG. 34), i.e. is moving towards        the front of helicopter 1, tip portion 11′″ undergoes a negative        lift; and    -   when blade 6′″ is retreating (FIG. 33), i.e. is moving towards        the rear of helicopter 1, tip portion 11″ undergoes a positive        lift.

Accordingly, aerodynamic moment Mlift acting on tip portion 11′″ ofblade 6′″ is directed in the first angular direction, clockwise in FIG.35, when blade 6 is advancing; and is directed in the second angulardirection, counter-clockwise in FIG. 35, when blade 6′″ is retreating.

Moments Mlift, Mk, Md, Mc generate a resulting moment Mr about axis ontip portions 11′″. Preferably, spring 90′″, ballasts 91′″ and damper 92″are configured to direct resulting moment Mr:

-   -   in the second angular direction, when helicopter 1 in in hover        or in forward flight with a speed lower than the threshold        value; and    -   cyclically in the first and second angular directions        (respectively if the blade is advancing or retreating), when        helicopter 1 is in forward flight with a speed greater than the        threshold value.

Depending on the orientation of resulting moment Mr and on theconfiguration of connecting means 60′″, tip portion 11′″ is rotated inthe first or in the second angular direction or remains angularly fixedwith respect to main portion 10.

In particular, in case resulting moment Mr is directed in the firstangular direction and connecting means 60′″ are set in the firstconfiguration, tip portion 11′″ is rotated up to the first angularposition.

In case resulting moment Mr is directed in the first angular directionand connecting means 60′″ are set in the second configuration, tipportion 11′″ is not rotated.

In case resulting moment Mr is directed in the second angular directionand connecting means 60′″ are set in the second configuration, tipportion 11′″ is rotated up to the second angular position.

In case resulting moment Mr is directed in the second angular directionand connecting means 60′″ are set in the first configuration, tipportion 11′″ is not rotated.

Furthermore, blade 6′″ comprises a partially relieved stop element 88′″,which defines a first and a second stop for tip portion 11′″ set in thefirst angular position and the second angular position respectively.

Preferably, stop element 88′″ stops tip portion 11′″ in the firstposition at anhedral angle α of about 20 degrees with respect to theplane of main portion 10 and in the second position at a null anhedralangle α with respect to the plane of main portion 10.

In greater detail, blade 6′″ comprises an actuator 65′″ controllable bya control unit 66′″ of rotor 3′″ and which can be operated to setconnecting means 60′″ in either the first configuration or the secondconfiguration.

In greater detail, actuator 65′″ is housed inside main portion 10.

Actuator 65″″ comprises (FIGS. 22 and 23 a):

-   -   an electric motor 70′″ controlled by control unit 66′″ and        having an output shaft 71′″ rotating parallel to axis F;    -   an output member 73′″ slidable parallel to axis F; and    -   a transmission group 72′″ interposed between electric motor 70′″        and output member 73′″.

In particular, transmission group 72′″ comprises:

-   -   a shaft 74′″ rotatable parallel to axis F, provided with a        worm-screw 75′″ and having a gear 76′″ at an end thereof;    -   a gear 77′″ meshing with gear 76′″ and arranged at an axial end        of output shaft 71′″; and    -   a slide 78′″, which is free to slide parallel to axis F,        comprises a rack 79′″ meshing with worm-screw 75′″ and is        integrally movable together with output member 73′″.

Output member 73′″ is housed in a compartment of main portion 10.

Connecting means 60′″ comprise, in turn:

-   -   a plurality of coupling elements 80′″ carried by main portion 10        and connected to output member 73′″; and    -   a plurality of coupling elements 81 a″′, 81 b′″ carried by tip        portion 11′″.

In particular, coupling elements 81 a′″ are arranged at axial ends oftip portions 11′″ with respect to axis F. Coupling elements 81 b′″ (onlyone of which is shown in FIGS. 22 and 23 a) are axially interposedbetween coupling elements 81 a′″ and are spaced along axis F

Coupling element 80′″ are also axially spaced along axis F.

In particular, each coupling element 80′″ is axially interposed betweenone adjacent coupling element 81 a′″ and an adjacent coupling element 81b′″, or between two adjacent coupling elements 81 b″′, or between one anadjacent coupling element 81 b′″ and other one adjacent coupling element81 a′″.

Actuator 65″′ can be operated to alternatively engage coupling elements80′″ with first adjacent coupling elements 81 a′″, 81 b′″ in the firstconfiguration of connecting means 60′″, or to engage coupling elements80′″ with second adjacent coupling elements 81 a′″, 81 b′″ in the secondconfiguration of connecting means 60′″.

Each coupling element 80′″ comprises, in turn,

-   -   a pair of axial end disks 82 a′″, 82 b′″ opposite to another and        having respective toothed surfaces 83 a′″, 83 b′″; and    -   a pair of one-way freewheel clutches 84′″, 85′″ axially        interposed between disks 82 a′″, 82 b′″.

In greater detail, surfaces 83 a′″, 83 b′″ are arranged on respectiveopposite axial sides of one-way freewheel clutches 84′″, 85′″.

Furthermore, one-way freewheel clutch 84′″ allows the rotation of disk82 a′″ only in the first angular direction and one-way freewheel clutch85′″ allows the rotation of disk 82 b′″ only in the second angulardirection.

Each coupling element 81 b′″ comprises a pair of axial end disks 86 a′″,86 b′″ opposite to another and having respective toothed surfaces 87a′″, 87 b′″.

Each coupling element 81 a′″ comprises only one axial end disk 86 a′″having respective toothed surface 87 a′″.

Surfaces 83 a′″, 83 b′″ of coupling element 80′″ face surfaces 87 a′″,87 b″′ of coupling elements 81 a′″, 81 b′″, which are axially adjacentthereto.

Actuator 65′″ can be operated to displace coupling elements 80′″ alongaxis F:

-   -   either up to a first axial position in which surfaces 83 a′″ of        disk 82 a′″ mesh with respective surfaces 87 a′″, 87 b′″ of disk        86 a′″ of first adjacent coupling elements 81 a′″, 81 b′″ so as        to determine the engagement of coupling element 80′″ and first        adjacent coupling element 81 a′″, 81 b′″;    -   or up to a second axial position in which surfaces 83 a″′ of        disk 82 a′″ mesh with respective surface 87 a′″. 87 b′″ of        second adjacent coupling elements 81 a′″, 81 b″″, so as to        determine the engagement of coupling element 80′″ and second        adjacent coupling element 81 a′″, 81 b′″.

In the first axial position, one-way freewheel clutches 84′″ allow tipportion 11′″ to rotate in the first angular direction relative to mainportion 10 and prevent tip portion 11′″ from rotating in the seconddirection relative to main portion 10.

In the second axial position, one-way freewheel clutches 85′″ allow tipportion 11′″ to rotate in the second angular direction relative to mainportion 10 and prevent tip portion 11′″ from rotating in the firstdirection relative to main portion 10.

In particular, teeth of toothed surface 83 a′″, 83 b′″ and teeth oftoothed surfaces, 87 a′″, 87 b′″ are shaped in such a way that whentoothed surfaces 83 a′″, 83 b″′ engage respective toothed surfaces, 87a′″, 87 b″′, disks 82 a″, 86 a″ and 82 h″, 86 h″ can rotate integrallywith one another about axis F, in both the first and the second angulardirections.

The operation of rotor 3′″ differs from that of rotor 3 in that theangular position of tip portion 11′″ with respect to main portion 10 isdetermined by the resulting moment Mr on tip portion 11′″ and by theposition of connecting means 60′″.

The operation of rotor 3′″ will be now described starting from a flightcondition, in which helicopter 1 is in hovering and with reference to asingle blade 6′″.

In this flight condition, aerodynamic moment Mlift is directed in thesecond direction when blade 6′″ is advancing or retroceding.

Furthermore, control unit 66′″ sets actuator 65′″ in the first axialposition. Accordingly, connecting means 60′″ are set in the firstconfiguration (shown in FIG. 22), in which they prevent the rotation oftip portion 11′″ in the second angular direction with respect to mainportion 10 and only allow the rotation of tip portion 11′″ in the firstangular direction with respect to main portion 10.

Accordingly, tip portion 11′″ is kept in the first angular position inwhich it defines anhedral angle α with respect to main portion 10.

Furthermore, tip portion 11′″ abuts against stop element 88′″, whichprevents any further rotation thereof in the first angular direction andany consequent undesired increase of anhedral angle α.

Being connecting means 60′″ set in the first configuration, couplingelements 80′″ engage coupling elements 81 a′″. Still more precisely,surfaces 83 b′″ of disks 82 b′″ engage surfaces 87 a′″, 87 b′″ of disk86 a′″, 86 b′″ of first adjacent connecting element 81 a′″, 81 b′″.Furthermore, one-way freewheel clutch 84″′ allows tip portion 11′″ torotate in the first angular direction relative to main portion 10 andprevents tip portion 11′″ from rotating in the second angular direction.

In case it is necessary to operate helicopter 1 in forward flight,control unit 66′″ sets actuator 65′″ in the second axial position. As aconsequence, connecting means 60′″ are also set in the secondconfiguration (FIG. 23a ), in which they allow the rotation of tipportion 11′″ with respect to main portion 10 in the second angulardirection and prevent the rotation of tip portion 11′″ in the firstangular direction.

Thanks to the fact that connecting means 60′″ are now set in the secondconfiguration, tip portion 11′″ increasingly rotates in the secondangular direction only, up to when it reaches the second angularposition in which the anhedral angle α is substantially null. In thesecond angular position, tip portion 11′″ abuts against stop element88′″ which prevents any further rotation thereof in the second angulardirection and any consequent undesired increase of the dihedral angle.

In particular, being connecting means 60′″ set in the secondconfiguration, control unit 66′″ sets actuator 65′″ in the second axialposition (FIG. 23a ).

Therefore, coupling elements 80′″ engage second adjacent couplingelements 81 a′″, 81 b′″. Still more precisely, surfaces 83 a′″ of disks82 a′″ engage surfaces 87 a′″, 87 b′″ of disk 86 a′″, 86 b′″ of secondadjacent coupling elements 81 a′″, 81 b′″. Furthermore, one-wayfreewheel clutches 85′″ allow tip portion 11′″ to rotate in the seconddirection relative to main portion 10 and prevent tip portion 11′″ fromrotating in the first direction.

In case it is necessary to operate helicopter 1 in hovering, controlunit 66′″ sets back actuator 65′″ in the first axial position. As aconsequence, connecting means 60′″ are also set in the firstconfiguration (FIG. 23a ), in which they allow the rotation of tipportion 11′″ with respect to main portion 10 in the first angulardirection only and prevent the rotation of tip portion 11′″ in thesecond angular direction.

The increase in the forward speed of helicopter 1 causes at a speedgreater than the threshold value:

-   -   a positive lift acting on tip portion 10′″ of blade 6′″ both        when it advances and retrocedes when the forward speed of        helicopter 1 is still lower than the threshold value; and    -   a positive lift acting on tip portion 10′″ of blade 6′″ when it        retrocedes and a negative lift acting on tip portion 10′″ of        blade 6′″ when it advances, when the forward speed of helicopter        1 is greater than the threshold value.

Accordingly, when the helicopter 1 still is in forward flight with aspeed greater than the threshold value, tip portion 11′″ rotates back inthe first angular direction up to when it reaches the first position.This is due to the fact that, being negative the lift on tip portion11′″ of advancing blade 6′″, resulting moment Mr on tip portion 11′″ isdirected in the first direction when blade 6′″ is advancing.

At this point, helicopter 1 is set in hovering.

Referring to FIGS. 36 to 38, reference numeral 3″″ indicates, as awhole, a rotor according to a fourth embodiment of the presentinvention.

Rotor 3″″ differs from rotor 3′″ in that each coupling element 80″″comprises, in turn:

-   -   an annular frame 95″″ connected to slide 78′″ and movable        parallel to axis F with slide 78″″;    -   a pair of axial end disks 82 a″″, 82 b″″ (86 a″″, 86 b″″)        opposite to another and having respective toothed surfaces 83        a″″, 83 b″″ (87 a′″, 87 b′″);    -   a shaft 99″″ coaxial to axis F; and    -   a pair of elements 96″″ arranged at respective axial ends of        shaft 99″″ and fixed to respective disks 82 a″″, 82 b″″ (86 a′″,        86 b′″).

In greater detail, teeth of toothed surface 83 a″″, 83 b″″ and teeth oftoothed surfaces 87 a″″, 87 b″″ are shaped in such a way that whentoothed surfaces 83 a″″, (83 b″″) engage respective toothed surface 87a″″ (87 b″″) of first (second) adjacent angular coupling element 81 a″″,81 b″″, the rotation of disk 86 a″″ (86 b″″) in the first (second)angular direction causes the intermittent meshing of toothed surfaces 87a″″, 87 b″″.

In particular, the intermittent meshing causes a back and forth movementof surfaces 83 a″″ (83 b″″) towards and away from surface 87 a″″ (87b″″) parallel to axis F. Furthermore, teeth of toothed surface 83 a″″,83 b″″ and teeth of toothed surfaces 87 a″″, 87 b″″ are shaped in such away that when toothed surfaces 83 a″″, (83 b″″) engage respectivetoothed surface 87 a″″ (87 b″″), the rotation of disk 86 a″″ (86 b″″) inthe second (first) angular direction is prevented.

In the embodiment shown, elements 96″″ are angularly movable integrallywith shaft 99″″ about axis F and are axially slidable relative to shaft99″″ along axis F.

In the embodiment shown, teeth of surfaces 83 a″″, 83 b″″ and 87 a″″, 87b″″ are saw-teeth shaped.

In the embodiment shown, elements 96″″ are splined on the respectivesides facing axis F while shaft 99″″ is splined on the opposite sidewith respect to axis F.

Furthermore, each coupling element 80″″ comprises, in turn, elasticmeans 100″″ interposed between frame 95″″ and elements 96″″. Elasticmeans 100″″ elastically load elements 96″″ and disks 82 a″″, 82 b″″towards adjacent disks 86 a″″, 86 b″″ of adjacent coupling element 81a″″, 81 b″″.

In detail, elastic means 100″″ comprises a plurality of springs 101″″,helical springs in the embodiment shown, interposed between frame 95″″and elements 96″″.

Springs 101″″ extend along respective axes parallel to axis F andangularly spaced about axis F.

The operation of rotor 3″″ differs from that of rotor 3′″ in that the,when connecting means 60″″ are set in the first configuration (FIG. 36),toothed surfaces 83 a″″ of disks 82 a″″ mesh with toothed surfaces 87a″″ of disks 86 a″″ while toothed surfaces 83 b″″ of disks 82 b″″ areaxially spaced from toothed surfaces 87 b″″ of disks 86 b″″.

Due to the shape of teeth of surfaces 83 a″″, 87 a″″, the meshingbetween disks 82 a″″, 86 a″″ allows the intermittent angular rotation ofdisks 86 a″″ and, therefore, of tip portion 11″″ in the first angulardirection with respect to disks 82 a″″ and, therefore, to main portion10.

This intermittent angular rotation is formed by a succession ofalternate first time intervals in which teeth of surfaces 83 a″″, 87 a″″mesh with one another and second time intervals in which teeth ofsurfaces 83 a″″, 87 a″″ are disengaged.

During the first time intervals, surfaces 83 a″″ rotate in the firstangular direction, thus rotating tip portions 11″″.

During the second time intervals, surfaces 83 a″″ do not rotate, and theaxial disengaging movement of surface 82 a″″ causes the axial movementof elements 96″″ away from surfaces 83 a″″ and the compression ofsprings 101″″. The subsequent extension of spring 101″″ causes themeshing of teeth of surfaces 83 a″″, 87 a″″ of respective disks 82 a″″,86 a″″.

When the connecting means 60″″ are set in the second angular position,teeth of surfaces 83 b″″, 87 b″″ mesh with one another, thus preventingthe rotation of disks 86 b′″ and, therefore, of tip portion 11 in thefirst angular direction and allowing the intermittent angular rotationof disks 86 b′″ and, therefore, of tip portion 11 in the second angularposition.

From examination of the characteristics of the rotor 3, 3′ 3″, 3′″ and3″″ and the method according to the present invention, the advantagesthat can be achieved therewith are evident.

In particular, the tip portion 11 of each blade 6 is selectively movablebetween:

-   -   the respective first position, in which the dihedral/anhedral        angle α with respect to the main portion 10 is non-zero (FIGS. 4        to 6); and    -   the respective second position, in which the sweep angle β with        respect to the main portion 10 is non-zero (FIGS. 7 to 9).

In this way, unlike the known types of rotors described in theintroductory part of this description, the rotor 3, 3′, 3″ and 3′″enable both:

-   -   having high aerodynamic efficiency when the helicopter 1 is in        hovering conditions and the blades 6 are arranged in the        associated first positions; and    -   having low noise when the helicopter 1 is in forward flight        conditions and the blades 6 are arranged in the associated        second positions.

This behaviour, optimized both in hovering conditions and in forwardflight conditions, is obtained in a particularly simple manner and withsubstantially zero displacement of the barycentre of the blades 6 alongaxes C and D between the respective first and second positions.

In this way, the motor 20 does not need to counter high loads due tocentrifugal force and only needs to generate relatively low actuatingforces. In consequence, the motor 20 can be compact and low-cost.

Due to the fact that the transition of the tip portions 11 and 11″ doesnot result in a substantial variation in the position of the barycentreof the associated blades 6 along axes C and D, the erroneous operationof one or more of the motors 20 and therefore the erroneous positioningof the respective tip portion 11 and 11″ does not cause inertialunbalancing of the rotor 3, 3′ and 3″.

Due to the fact that the motor 20 is housed in the main portion 10 andthat the sections 25 and 26 of the rods 21 are housed in the tipportions 11, the rotor 3, 3′ and 3″ does not alter the aerodynamicprofile of the blades 6 and does not penalize the overall aerodynamicefficiency of the helicopter 1.

Finally, connecting means 60′″, 60″″ can be selectively set in a first(second) configuration in which they allow the rotation of tip portion11′″ in the first (or in the second) angular direction and up to thefirst (second) angular position, and in which they prevent tip portion11′″, 11″″ from rotating in the second (or in the first) angulardirection with respect to main portion 10.

In this way, it possible to move tip portions 11′″, 11″″ between thefirst and second angular positions, by taking advantage of the fact thatresulting Mr on tip portions 11′″, 11″″:

-   -   is directed in the second angular direction when helicopter 1 is        in hovering or in forward flight with a speed lower than        threshold value; and    -   is directed in the first angular direction when relative blades        6′″ are advancing and helicopter 1 is in forward flight with a        speed greater than the threshold value.

As matter of fact, when it is necessary to move tip portions 11′″, 11″″in the first angular position with the aim of increasing the anhedralangle of thereof, it is enough that actuator 65′″ sets connecting means60′″, 60″″ in the first configuration and that the forward speed of thehelicopter 1 is greater than the threshold value.

Conversely, when it necessary to move tip portions 11′″, 11″″ in thesecond angular position with the aim of reducing anhedral angle αthereof, it is enough that actuator 65′″ sets connecting means 60′″,60″″ in the second configuration for any hovering condition or forwardspeed of helicopter 1.

Accordingly, it is possible to adjust the anhedral angle of tip portions11′″, 11″″ with respect to main portion 10, by simply relying onaerodynamic forces and preferably on elastic moment Mk provided byspring 90′″ and/or damping moment Md provided by damper 92′″.

It is therefore no longer necessary to provide a dedicated actuatingsystem for adjusting the dihedral/anhedral angle of tip portions 11′″,11″″ with respect to main portion 10.

This is particularly advantageous because there is no need to transferlarge power to rotating blades 6′″.

Furthermore, there is no need to provide the necessary volume for theactuating system at the interface between main portion 10 and tipportions 11′″, 11″″. The latter can be therefore made in an optimizedshape to meet the aerodynamic requirements.

Finally, it is also clear that modifications and variants can be maderegarding the rotor 3, 3′, 3″, 3′″ and 3″″ and method described andillustrated herein without departing from defined by the claims.

In particular, the rotor 3, 3′ 3″, 3′″ and 3″″ could be used in aconvertiplane instead of in the helicopter 1.

Furthermore, the tip portions 11 and 11″ could have dihedral angles α inthe respective first positions and negative sweep angles β in therespective second positions.

The transmission unit 22 could comprise an epicyclical train interposedbetween the motor 20 and the rods 21.

The actuator units 16 could be completely housed inside the tip portions11 of the respective blades 6.

Rotor 3′ might not comprise the transmission unit 22 and could comprisea plurality of motors 20 connected to associated rods 21 and notcomprise the transmission unit 22.

Finally, as regard to rotor 3′″, 3″″, the first and the second angularpositions of tip portions 11′″, 11″″ could correspond also to dihedralangles with respect to main portion 10 and could be achieved in flightconditions others than the ones indicated in the present description.

The invention claimed is:
 1. A rotor (3′″, 3″″) for a hover-capableaircraft (1), comprising: a hub (7) rotatable about a first axis (A);and at least two blades (6′″) hinged to said hub (7); each of said atleast two blades (6′″) comprising a main portion (10) articulated tosaid hub (7) and a tip portion (11′″, 11″″), which is arranged radiallyoutermost from said first axis (A) with respect to the correspondingmain portion (10); said tip portion (11′″, 11″″) of each of said atleast two blades (6′″) being movable with respect to the correspondingsaid main portion (10) of said at least two blades (6′″) between: afirst angular position, in which said tip portion defines a firstdihedral or anhedral angle (α) with respect to the corresponding saidmain portion (10); and a second angular position, in which said tipportion defines a second dihedral or anhedral or null angle differentfrom the first dihedral or anhedral angle (α) with respect to thecorresponding said main portion (10); said rotor (3′″, 3″″) furthercomprising connecting means (60′″, 60″″) for movably connecting said tipportion (11′″, 11″″) to said main portion (10); characterized in thatsaid connecting means (60′″, 60″″) can be selectively set in a firstconfiguration in which said connecting means (60′″, 60″″): allow therotation of said tip portion (11′″, 11″″) with respect to said mainportion (10) in a first angular direction and up to said first angularposition of said tip portion (11′″, 11″″) with respect to said mainportion (10), by means of resultant moments (Mr) generated by inertiaforces and aerodynamic forces on said tip portion (11′″, 11″″); andprevent said tip portion (11′″, 11″″) from rotating in a second angulardirection, opposite to said first angular direction, with respect tosaid main portion (10).
 2. The rotor of claim 1, wherein said connectingmeans (60′″, 60″″) can be selectively set in a second configuration, inwhich said connecting means (60′″, 60″″): allow the rotation of said tipportion (11′″, 11″″) with respect to said main portion (10) in saidsecond angular direction and up to said second angular position of saidtip portion (10); and prevent said tip portion (11′″, 11″″) fromrotating with respect to said main portion (10) in said first angulardirection.
 3. The rotor of claim 2, further comprising: a control unit(66′″); and an actuator (65′″), which is controllable by said controlunit (66′″) and is actuatable to set said connecting means (60′″, 60″″)in either said first configuration or said second configuration.
 4. Therotor of claim 3, wherein said actuator (65′″) is controllable to setsaid connecting means (60′″, 60″″) in said first configuration, whensaid hover-capable aircraft (1) requires to be operated, in use, inhovering; said actuator (65′″) being controllable to set said connectingmeans (60′″, 60″″) in said second configuration when said hover-capableaircraft (1) requires to be operated, in forward flight.
 5. The rotor ofclaim 3, wherein said tip portion (11′″, 11″″) defines the firstanhedral angle (α) with respect to the corresponding said main portion(10) when set, in use, in said first angular position and/or said nullangle with respect to the corresponding said main portion (10) when set,in use, in said second angular position.
 6. The rotor of claim 5,wherein said actuator (65′″) comprises an output member (73′″), which isslidable back and forth parallel to a second axis (F).
 7. The rotor ofclaim 2, wherein said connecting means (60′″, 60″″) comprise a hinge(61′″), which is configured to allow the rotation of said tip portion(11′″, 11″″) with respect to said main portion (10) about a second axis(F).
 8. The rotor of claim 7, wherein said connecting means (60′″, 60″″)comprise: at least one first coupling element (80′″, 80″″) carried byone (11′″, 11″″) of said tip portion (11′″, 11″″) and said main portion(10); and at least one second coupling element (81 a′″, 81 b′″; 81 a″″,81 b″″) carried by the other (11′″, 11″) of said tip portion (11′″,11″″) and said main portion (10); said at least one first and at leastone second coupling elements (80′″, 80″″; 81 a′″, 81 b′″; 81 a″″, 81b″″) being selectively movable with respect to one another and parallelto said second axis (F) between a first axial position in which said atleast one first and at least one second coupling elements (80′″, 80″″;81 a′″, 81 b′″; 81 a″″, 81 b″″) are engaged with one another and asecond axial position in which said at least one first and at least onesecond coupling elements (80′″, 80″″; 81 a′″, 81 b′″; 81 a″″, 81 b″″)are disengaged.
 9. The rotor of claim 8, wherein said at least one firstand at least one second coupling elements (80′″; 81 a″″, 81 b″″)comprise respective first and second disks (82 a′″, 82 b″″; 86 a′″, 86b″″), which comprise respective plurality of first and second toothedsurfaces (83 a′″, 83 b′″; 87 a′″; 87 b′″) meshing with one another, whensaid at least one first and at least one second coupling elements (80″″;81 a″″, 81 b″″) are set, in use, in said first axial position; saidplurality of first and second toothed surfaces (87 a′″, 87 b′″) beingshaped in such a way to allow the rotation of said at least one firstand at least one second coupling elements (80′″, 80″″; 81 a′″, 81 b′″;81 a′″, 81 b′″) both in said first angular direction and in said secondangular direction; said at least one first coupling element (80′″)comprising at least one one-way free-wheel clutch (84′″, 85′″), which isinterposed between said first disk (82 a′″, 82 b′″; 86 a′″, 86 b′″) andsaid relative one (10) of said tip portion (11′″) and said main portion(10).
 10. The rotor of claim 8, wherein said at least one first and atleast one second coupling elements (80″″; 81 a″″, 81 b″″) compriserespective first and second disks (82 a″″, 82 b″″; 86 a″″, 86 b″″),which comprise respective plurality of first and second toothed surfaces(83 a″″, 83 b″″; 87 a″″; 87 b″″) meshing with one another, when said atleast one first and at least one second coupling elements (80″″; 81 a″″,81 b″″) are set, in use, in said first axial position; said plurality offirst and second toothed surfaces (83 a″″, 83 b″″; 87 a″″; 87 b″″) beingshaped in such a way to allow the intermittent rotation of said at leastone first and at least one second coupling elements (80″″; 81 a″″, 81b″″) in a respective one of said first angular direction and secondangular direction and to prevent that rotation in respective another ofsaid first direction and second direction.
 11. The rotor of claim 10,wherein first and second teeth of said plurality of first and secondtoothed surfaces (87 a″″, 87 b″″) are saw-tooth shaped.
 12. The rotor ofclaim 10, wherein said at least one first coupling element (80″″)comprises, in turn: a first splined body (95″″) movable about saidsecond axis (F); and at least one second splined body (96″″), which isangularly and axially movable integrally together with said first disk(82 a″″, 82 b″″); said at least one second splined body (96″″) beingaxially movable and angularly integral with respect to said firstsplined body (95″″).
 13. The rotor of claim 12, further comprising: acontrol unit (66′″); and an actuator (65′″), which is controllable bysaid control unit (66′″); wherein said actuator (65′″) comprises anoutput member (73′″), which is slidable back and forth parallel to thesecond axis (F), and the rotor further includes first elastic means(100″″, 101″″) interposed between said output member (73″″) and said atleast one second splined body (96″″), and configured to elastically loadsaid at least one second splined body (96″″) towards said first axialposition.
 14. The rotor of claim 13, further comprising second elasticmeans (90′″), which are interposed between said main portion (10) andsaid tip portion (11′″, 11″″) and are configured to elastically load, inuse, said tip portion (11′″, 11″″) towards one of said first or secondangular position.
 15. The rotor of claim 8, further comprising: anactuator (65′″) with an output member (73′″); wherein said at least onefirst coupling element (80″″) is carried by said main portion (10) andsaid at least one second coupling element (81 a″″, 81 b″″) is carried bysaid tip portion (11′″, 11″″); said at least one first coupling element(80″″) being movable together with said output member (73″″) parallel tosaid second axis (F).
 16. The rotor of claim 1, further comprisingdamping means (91′″) which are interposed between said main portion (10)and said tip portion (11′″, 11″″) and are configured to exert, in use, adamping moment (Md) on said tip portion (11′″, 11″″).
 17. The rotor ofclaim 1, wherein said main portion (10) comprises at least one ballast(91′″).
 18. A method of controlling a rotor (3′″, 3″″) for ahover-capable aircraft (1); said rotor (3′″, 3″″), comprising: a hub (7)rotatable about a first axis (A); and at least two blades (6) hinged tosaid hub (7); each of said at least two blades (6) comprising a mainportion (10) hinged to said hub (7) and a tip portion (11′″, 11″″),which is arranged radially outermost with respect to said first axis (A)with respect to the corresponding main portion (10); said methodcomprising the steps of: i) connecting said tip portion (11′″, 11″″) inmovable way with respect to said main portion (10); ii) moving said tipportion (11′″, 11″″) with respect to said main portion (10) between: afirst angular position, in which said tip portion defines a firstdihedral or anhedral angle (α) with respect to the corresponding saidmain portion (10); and a second angular position, in which said tipportion defines a second dihedral or anhedral angle (α) or a null angle,different from said first dihedral or anhedral angle (α), with respectto the corresponding said main portion (10); said method beingcharacterized by comprising the steps of selectively: iii) allowing therotation of said tip portion (11′″, 11″″) with respect to said mainportion (10) in a first angular direction and up to said first angularposition, by means of resultant moments (Mr) generated by inertia forcesand aerodynamic forces on said tip portion (11′″, 11″″); and iv)preventing said tip portion (11′″, 11″″) from rotating in a secondangular direction, opposite to said first angular direction, withrespect to said main portion (10).
 19. The method of claim 18, furthercomprising the further steps of selectively: v) allowing the rotation ofsaid tip portion (11′″, 11″″) with respect to said main portion (10) insaid second angular direction and up to said second angular position ofsaid tip portion (11′″, 11″″); and vi) preventing said tip portion(11′″, 11″″) from rotating with respect to said main portion (10) insaid first angular direction.
 20. The method of claim 19, wherein saidstep v) is carried out when said hover-capable aircraft (1) is inforward flight with a speed greater than a threshold value and saidhover-capable aircraft (1) needs to be operated in hover and/or whensaid hover-capable aircraft (1) is in hover; and/or said step vi) iscarried out when said hover-capable aircraft (1) is in hover and saidhover-capable aircraft (1) needs to be operated in forward flight. 21.The method of claim 19, further comprising the steps of: vii) keepingsaid tip portion (10′″, 10″″) in said first angular position, when saidhover-capable aircraft (1) is in hover and said hover-capable aircraft(1) needs to be operated in said hover; and/or viii) keeping said tipportion (10′″, 10″″) in said second angular position, when saidhover-capable aircraft (1) is in forward flight and said hover-capableaircraft (1) needs to be operated in said forward flight.
 22. The methodof claim 19, further comprising at least one of the steps ix) ofgenerating an elastic moment (Mk) on said tip portion (10′″, 10″″) andx) of generating a damping moment (Md) on said tip portion (10′″, 10″″);a lift acting on said tip portion (11′″, 11″″) generating an aerodynamicmoment (Mlift) being directed in said second angular direction when saidhover-capable aircraft (1) is in hover or in forward flight with a speedlower than a threshold value; said aerodynamic moment (Mlift) beingdirected in said first angular direction when said hover-capableaircraft (1) is in forward flight with a speed greater than saidthreshold value and at least for one angular position of said at leasttwo blades (6′″, 6″″) with respect to said first axis (A); wherein oneresulting moment (Mr) of said resultant moments (Mr) on said tip portion(10′″, 10″″) being directed in said second angular direction when saidhover-capable aircraft (1) is in hover or in forward flight with a speedlower than said threshold value, and being directed in said firstangular direction when said hover-capable aircraft (1) is in forwardflight with a speed greater than said threshold value and at least forsaid one angular position of said at least two blades (6′″, 6″″) withrespect to said first axis (A).
 23. A hover-capable aircraft,comprising: a main rotor (3, 3′, 3″) according to claim 1; and a controlunit (40) programmed to arrange said tip portions (11′″, 11″″) of saidat least two blades (6′″) in the respective said first angular positionswhen said hover-capable aircraft (1) is in hovering conditions and inthe respective said second angular positions when said hover-capableaircraft (1) is in forward flight conditions.